Lateral semiconductor device and method of fixing potential of the same

ABSTRACT

A lateral semiconductor device with enhanced breakdown characteristics includes a semiconductor substrate composite of first and second semiconductor substrates bonded to one another via an oxide film. An insulation film is buried in a separation trench which extends from a major surface of the first semiconductor substrate to the oxide film. An element region of 10 μm or more in thickness is isolated by the separation trench from other element regions. First and second diffusion regions of opposite conductivity type are formed on the element region. The potential of the second substrate is fixed at one-third of the designed maximum breakdown voltage of the lateral semiconductor device. Alternatively, if the element region is 10 μm or less in thickness, the potential of the second substrate is fixed at one-half of the designed maximum breakdown voltage of the lateral semiconductor device.

BACKGROUND OF THE INVENTION

The present invention relates to lateral semiconductor devices, and to methods of driving such devices.

In lateral semiconductor devices, carriers flow parallel to major device surfaces, and signals are input and output through electrodes disposed on one major surface. Such lateral semiconductor devices may be contrasted with vertical semiconductor devices in which the carriers flow perpendicularly to the semiconductor substrate. The latter are used especially as so-called power devices, i.e., semiconductor devices for handling electric power in high-power semiconductor apparatuses.

Vertical semiconductor devices have been used mainly based on breakdown voltage considerations. The breakdown voltage V_(B) of a semiconductor device may be expressed by Equation 1:

    V.sub.B =1/2·E.sub.Crit ·L.sub.D         ( 1)

where L_(D) is "drift length" or width of a depletion layer when a voltage is applied across a junction, and E_(Crit) is the critical electric field strength of the junction. When the electric field strength inside the device exceeds the critical strength E_(Crit), breakdown occurs. The value of E_(Crit) depends on the shape and method of forming the junction, and on impurity concentration and other factors. Mainly, however, it depends on the drift length L_(D). With adequate thickness, and without requiring excessive lateral chip size, the vertical device is superior for high-breakdown-voltage devices because it permits greater expansion of a depletion layer.

Recently, single-chip power ICs having power devices and an IC monolithically integrated have attracted increased attention. In order to match the manufacturing process of the power devices to that of the IC, such power devices are being configured in lateral form.

FIG. 6 is a cross section of a power IC with p-n junction separation. The power IC has a p-type substrate 1 on which an element region 2 is formed by epitaxial growth. The element region 2 is isolated by a p-type separation region 3 which surrounds the element region 2 from its surface down to the substrate 1. A p-type diffusion region 4 and an n-type diffusion region 5 corresponding respectively to a collector region and an emitter region of a bipolar transistor are formed in the element region 2. Electrodes respectively connected to terminals C and E are disposed on the diffusion regions 4 and 5. A bias voltage V_(CE) is applied between the terminals C and E. Usually, the minimum potential of the power supply voltage of the power IC is applied to the p-type substrate 1. For example, when the power supply voltage is ±15 V, a voltage of -15 V is applied to the substrate 1. Or, when the power supply voltage is +15 V, a bias voltage of 0 V (GND) is applied to the substrate 1.

In FIG. 6, the negative terminal of the bias power supply V_(CE) and an S terminal of the substrate 1 are grounded, and the potential of the substrate 1 is fixed at 0 V (GND). Such biasing creates a reverse-biased junction between the p-type substrate 1 and the n-type diffusion region 5, thereby isolating the substrate 1 from the element region 2 by a depletion layer. This biasing scheme is disclosed in Japanese Patent Publication No. S40-17410.

Two major drawbacks of the conventional p-n junction separation and biasing scheme described above are the parasitic-element effect and the limited breakdown voltage of an element. Though the p-type substrate 1 and the p-type separation region 3 are fixed at the minimum potential of the element region 2, they may form a pnp parasitic transistor, for example, causing thyristor or latch-up operation of the element.

To improve the breakdown voltage of the ICs which employ p-n junction separation, the thickness of the n-type epitaxial layer in the element region 2 must be increased, as equation 1 indicates. However, as the thickness of the epitaxial layer increases, the p-type separation region 3 should be diffused more deeply. This deep diffusion causes wide lateral diffusion, using up lateral device area. Consequently, it is difficult to achieve high-breakdown voltage in the device.

To avoid these drawbacks, new techniques have been sought for increasing the breakdown voltage at reduced area requirements for lateral element separation. A so-called "perfect dielectric separation structure" has been proposed that combines a semiconductor substrate composite including semiconductor substrates bonded with one another via an oxide film. Such semiconductor devices, and a method for increasing the breakdown voltage of the semiconductor device are disclosed in Japanese Laid Open Patent Application No. H04-336446 and in European Patent Publication No. 0513764 A2 (hereinafter referred to as "cited prior art").

FIG. 7 is a cross section of a part of the perfect dielectric separation-type semiconductor device disclosed in the cited prior art. The semiconductor device includes a semiconductor substrate composite including a first semiconductor substrate 6 and a second semiconductor substrate 7 bonded with one another via an oxide film 8. A separation trench 9 in which insulator is buried is dug from the surface of the substrate 6 down to the oxide film 8. A p-type diffusion region 11 and an n-type diffusion region 12 are formed in an element region 10 isolated from other element regions. The breakdown voltage V_(B) of the semiconductor device of FIG. 7 is thereby improved by fixing the potential V_(S) of the second semiconductor substrate 7 to be higher than the minimum potential inside the element region 10 formed in the first substrate 6, i.e., the ground potential connected to the p-type diffusion region 11.

Though the conventional method of increasing a device breakdown voltage of the semiconductor device shown in FIG. 7 may be useful, the semiconductor device cannot be manufactured in accordance with desired specifications as no method of designing the breakdown voltage has been known. The cited prior art points to the problem. For example, it mentions that the substrate potential is determined "by a trial approach", and that the substrate potential which maximizes the breakdown voltage can be determined "once the semiconductor device is determined."

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductor device with maximized breakdown voltage. It is a further object of the invention to provide a method of fixing the potential of the semiconductor device that is best suited for improving the device breakdown voltage.

According to an aspect of the invention, a lateral semiconductor device comprises a semiconductor substrate composite comprising a first semiconductor substrate of thickness d and a second semiconductor substrate bonded with one another via an oxide film, a separation trench having an insulation film buried therein, the separation trench extending from a major surface of the first semiconductor substrate to the oxide film, an element region isolated by the separation trench from other element regions, a first diffusion region of a first conductivity type formed on the element region, spaced from the separation trench by a distance L_(G), and a second diffusion region of a second conductivity type formed on the element region, spaced from the separation trench by a distance L_(G), and spaced from the first diffusion region by a distance L_(D), where the thickness and the distances are such that L_(G) ≧(L_(D) -d).

According to a further aspect of the invention, a method is provided for fixing a potential of a lateral semiconductor device comprising a semiconductor substrate composite comprising a first semiconductor substrate and a second semiconductor substrate bonded with one another via an oxide film, a separation trench having an insulation film buried therein, the separation trench extending from a major surface of the first semiconductor substrate to the oxide film, an element region of 10 μm or more in thickness thereof, the element region being isolated by the separation trench from other element regions, a first diffusion region of a first conductivity type formed on the element region, and a second diffusion region of a second conductivity type formed on the element region, the method comprising a step of fixing the potential of the second substrate to be one-third of the maximum breakdown voltage of the lateral semiconductor device.

According to still another aspect of the invention, a method is provided for fixing a potential of a lateral semiconductor device comprising a semiconductor substrate composite comprising a first semiconductor substrate and a second semiconductor substrate bonded with one another via an oxide film, a separation trench having an insulation film buried therein, the separation trench extending from a major surface of the first semiconductor substrate to the oxide film, an element region of 10 μm or less in thickness thereof, the element region being isolated by the separation trench from other element regions, a first diffusion region of a first conductivity type formed on the element region, and a second diffusion region of a second conductivity type formed on the element region, the method comprising a step of fixing the potential of the second substrate to be one-half of the maximum breakdown voltage of the lateral semiconductor device.

It is preferred to bury polycrystalline silicon in the separation trench, and to apply a potential bias higher than the minimum potential of the element region to the polycrystalline silicon. It is preferred also to a apply the same potential bias to the polycrystalline silicon as to the second substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail hereinafter with reference to the accompanied drawings which illustrate embodiments of the invention, wherein:

FIG. 1 is a cross section of a first embodiment of a perfect dielectric separation-type semiconductor device according to the invention.

FIG. 2 is a cross section of a second embodiment of a perfect dielectric separation-type semiconductor device according to the invention.

FIG. 3 is a graph relating breakdown voltage of the device to the potential of the second semiconductor substrate.

FIG. 4 is a graph relating distance between the n-type diffusion region and the separation trench to the breakdown voltage of the device of FIG. 1.

FIG. 5(a) is a graph showing potential distribution in the semiconductor device of FIG. 2, in which a potential bias is not applied to the polycrystalline silicon.

FIG. 5(b) is a graph showing potential distribution in the semiconductor device of FIG. 2, in which the same potential bias is applied to the polycrystalline silicon as to the second substrate 7.

FIG. 6 is a cross section of a prior-art p-n junction separation-type semiconductor device.

FIG. 7 is a cross section of a part of a prior-art perfect dielectric separation-type semiconductor device.

DETAILED DESCRIPTION

The influence of the substrate potential V_(S) on the device structure may be described as follows. When a junction exists in the element region 10, the expansion width X_(D) of the depletion layer expanding from the oxide film 8 is expressed by Equation 2:

    X.sub.D =[2ε.sub.Si ((V.sub.B -V.sub.S)+2|φ.sub.Fn |)(qN.sub.D).sup.-1 ].sup.1/2 (2)                (2)

where q is the electron charge, N_(D) is the impurity concentration in the first semiconductor substrate 6, ε_(Si) is the dielectric constant of silicon, φ_(Fn) is the Fermi potential of the element region, V_(B) is the reverse bias voltage (breakdown voltage), and V_(S) is the substrate potential of the second semiconductor substrate 7.

Mechanism 1. By Equation 2, applying V_(S) reduces the expansion width X_(D) of the depletion layer. Accordingly, applying V_(S) increases the breakdown voltage by V_(S) with respect to the breakdown voltage V_(B0) at zero substrate potential (V_(S) =0). That is, ##EQU1## where d is the thickness of the first semiconductor substrate 6.

Mechanism 2. The electric field strength of the depletion layer is at a maximum at the corner of the depletion layer. In the structure of FIG. 7 especially, the electric field strength may be related to the electric field across the oxide film 8. When a potential bias V_(S) is applied to the second semiconductor substrate 7, the electric field strength in the vicinity of the n-type diffusion region 12 is expressed by Equation 4, and the electric field strength in the vicinity of the p-type diffusion region 11 by Equation 5:

    E.sub.Crit (n)=α(V.sub.B *-V.sub.S)/d.sub.OX         (4)

    E.sub.Crit (p)=α·V.sub.S /d.sub.OX          (5)

where α is a geometrical correction factor, V_(B) * is a breakdown voltage according to Mechanism 2, and d_(OX) is the thickness of the oxide film 8 between the first and second semiconductor substrates 6 and 7. Equations 4 and 5 show that the potential V_(S) of the second semiconductor substrate determines whether the vicinity or the n-type diffusion region 12 or the vicinity of the p-type diffusion region 11 determines the breakdown voltage.

Mechanism 1 corresponds to the case in which the thickness d of the first semiconductor substrate 6 determines the breakdown voltage. Mechanism 2 corresponds to the case in which the thickness d_(OX) of the oxide film and the radius of curvature of the diffusion region 11 or 12 determine the breakdown voltage.

In the case of Mechanism 1, V_(B) ≦V_(B) * holds. Therefore

    E.sub.Crit (n)d.sub.OX /α-E.sub.Crit ·d+qN.sub.D d.sup.2 /(2ε.sub.Si)≧0                             (6)

Generally, the thickness d of the first semiconductor substrate that satisfies Equation 6 is d≦10 μm. In the usual case in which the substrate impurity concentration N_(D) is low, the maximum breakdown voltage then is given as follows: ##EQU2##

Since the maximum breakdown voltage occurs when V_(B) =V_(B) *, V_(B) * in Equation 4 is substituted by V_(B) :

    E.sub.Crit (n)d.sub.OX /α=E.sub.Crit ·d

    α/d.sub.OX =E.sub.Crit (n)/(E.sub.Crit ·d)

Since, typically, E_(Crit) (n)=E_(Crit) (p):

    V.sub.s =E.sub.Crit ·d

by Equation 5. Therefore, by Equation 3:

    V.sub.B =2V.sub.S                                          (7)

On the other hand, if the breakdown voltage is determined by Mechanism 2, a relation V_(B) ≧V_(B) * holds. Accordingly,

    E.sub.Crit (n)d.sub.OX /α-E.sub.Crit ·d+qN.sub.D d.sup.2 /(2ε.sub.Si)≦0                             (8)

Generally, the thickness d of the first semiconductor substrate that satisfies Equation 8 is d≧10 μm. Accordingly, in the case of Mechanism 2, the maximum breakdown voltage is determined by E_(Crit) (n)=E_(Crit) (p), and

    V.sub.B *=2V.sub.S

    V.sub.S =1/2V.sub.B *

The maximum breakdown voltage then is given as follows: ##EQU3##

Since the first semiconductor substrate is fully depleted when the breakdown voltage is determined by Mechanism 2, the breakdown voltage is determined by the potential concentrated in the oxide film 8:

    V.sub.B *=E.sub.Crit ·d

    V.sub.Bmax =3V.sub.S                                       (9)

Hereinafter, the case in which the maximum breakdown voltage is determined by Mechanism 1 is referred to as "double effect", and the case in which the maximum breakdown voltage is determined by Mechanism 2 as "triple effect".

By setting the spacing L_(G) between the diffusion regions and the insulation trench, the thickness d of the first semiconductor substrate, and the spacing L_(D) between the diffusion regions so that the condition L_(G) ≧(L_(D) -d) is met, the breakdown voltage of the dielectric separation-type semiconductor device is improved.

If an element region of 10 μm or more in thickness is formed on the second semiconductor substrate, breakdown voltage of the device is maximized by applying a potential to the second semiconductor substrate which is one-third of the designed maximum breakdown voltage of the device. If an element region of 10 μm or less in thickness is formed on the second semiconductor substrate, breakdown voltage of the device is maximized by applying a potential to the second semiconductor substrate which is one-half of the designed maximum breakdown voltage of the device.

The breakdown voltage of the dielectric separation type semiconductor device is improved also by filling the separation trench with polycrystalline silicon, and by applying a potential bias higher than the minimum potential of the element region, e.g., as high as the potential of the semiconductor substrate.

FIG. 1 is a cross section of a first embodiment of a perfect dielectric separation-type semiconductor device according to the invention. The semiconductor device of FIG. 1 includes a semiconductor substrate composite including a first semiconductor substrate 6 and a second semiconductor substrate 7 bonded with one another via an oxide film 8. A separation trench 9 in which insulator is buried is dug from the surface of the first substrate 6 down to the oxide film 8. A p-type diffusion region 11 and an n-type diffusion region 12 are formed in an element region 10 isolated from other element regions. The breakdown voltage V_(B) is improved by fixing the potential V_(S) of the second semiconductor substrate 7 to be higher than the minimum potential inside the element region 10 formed in the first substrate 6, i.e., higher than the ground potential connected to the p-type diffusion region 11.

The parameters of an experimental embodiment in accordance with FIG. 1 are as follows: the thickness d of the first semiconductor substrate 6 is 10 μm or 30 μm, the impurity concentration of the first semiconductor substrate is 1×10¹⁴ cm⁻³ (n-type), the dose amount of the p-type diffusion region is 1×10¹⁵ cm⁻², and the dose amount of the n-type diffusion region is 3.1×10¹⁵ cm⁻². The diffusion depth of the p-type diffusion region 11 is set at 1.5 μm and 3.5 μm for investigating the effect of the radius of curvature of the diffusion regions. And the drift length L_(D) between the diffusion regions 11 and 12 is set at a constant 70 μm.

FIG. 3 is a graph relating breakdown voltage V_(B) of the device to potential V_(S) of the second semiconductor substrate. Though the breakdown voltage V_(B) increases at first with increasing substrate potential V_(S), V_(B) shows a peak, and decreases as V_(S) increases further. When the thickness of the first substrate is 10 μm (indicated by Δ symbols), the value of the substrate potential V_(S) at which the breakdown voltage V_(B) shows a peak corresponds to one-half of the peak value of the breakdown voltage V_(B). When the thickness of the first substrate is 30 μm (indicated by □ or ◯ symbols), the value of the substrate potential V_(S) at which the breakdown voltage V_(B) shows a peak corresponds to one-third of the peak value of the breakdown voltage V_(B). These experimental results are in good agreement with the theoretical prediction. Accordingly, instead of designing the breakdown voltage of the device after setting the substrate potential as in the prior art, the dimensions of the device can be determined after selecting the breakdown voltage of the device.

FIG. 4 is a graph relating distance L_(G) between the n-type diffusion region 12 and the separation trench 9 to breakdown voltage V_(B) of the device of FIG. 1. The breakdown voltage V_(B) increases as distance L_(G) increases and almost saturates beyond a certain distance (40 μm in this case). Repeated experiments conducted by the present inventor have revealed a condition expressed by the following equation under which a high-breakdown voltage is realized in the device structure of FIG. 1:

    L.sub.G ≧L.sub.D -d                                 (10)

Thus, the breakdown voltage of the dielectric separation-type semiconductor device is improved by setting the distance L_(G) between the diffusion regions and the insulation trench, the thickness d of the first semiconductor substrate, and the drift length L_(D) between the diffusion regions so that the condition L_(G) ≧(L_(D) -d) is met.

FIG. 2 is a cross section of a second preferred embodiment of a perfect dielectric separation-type semiconductor device. The second embodiment differs from the first embodiment in that polycrystalline silicon 13 is inserted into the separation trench 9. Additionally, a potential bias V_(G) is applied to the polycrystalline silicon 13. The value of the potential bias V_(G) may be the same as or different from the value of the potential V_(S) of the second substrate 7.

FIG. 5(a) is a graph showing a potential distribution in the semiconductor device of FIG. 2, in which a potential bias is not applied to the polycrystalline silicon 13.

FIG. 5(b) is a graph showing a potential distribution in the semiconductor device of FIG. 2, in which a potential bias V_(G) equal to the potential V_(S) of the second substrate 7 is applied to the polycrystalline silicon 13. The curves in the figures represent iso-potential curves drawn at increments of 50 V. The potential bias V_(G) applied to the polycrystalline silicon 13 functions similarly as the potential bias V_(S) applied to the second substrate 7. The potential bias V_(G) applied to the polycrystalline silicon 13 moderates the potential gradient, and is especially effective to realize a high-breakdown voltage when the condition of Equation 10 is not satisfied. 

I claim:
 1. A lateral semiconductor device comprising a semiconductor substrate composite comprising first and second semiconductor substrates mutually bonded via an oxide film, whereinthe first semiconductor substrate, having a thickness d, comprises an element region which is isolated from other element regions by an insulator region extending from a major surface of the first semiconductor substrate to the oxide film; a first diffusion region of a first conductivity type is disposed on the element region, spaced from the insulator region by a spacing L_(G) ; a second diffusion region of a second conductivity type is disposed on the element region, spaced from the insulator region by the spacing L_(G), and spaced from the first diffusion region by a spacing L_(D), and the thickness d and the spacings L_(D) and L_(G) are such that L_(G) ≧(L_(D) -d).
 2. The device of claim 1, wherein the element region has a thickness of 10 μm or less, wherein the device has a maximum breakdown voltage, and wherein the second substrate is at a potential which is one-half of the maximum breakdown voltage.
 3. The device of claim 2, wherein the element region has a minimum potential, wherein the separation region in part consists of polycrystalline silicon, and wherein the polycrystalline silicon is at a potential bias higher than the minimum potential of the element region.
 4. The device of claim 3, wherein the potential bias of the polycrystalline silicon is equal to a potential at which the second semiconductor substrate is held.
 5. The device of claim 1, wherein the element region has a thickness of 10 μm or more, wherein the device has a maximum breakdown voltage, and wherein the second substrate is at a potential which is one-third of the maximum breakdown voltage.
 6. The device of claim 5, wherein the element region has a minimum potential, wherein the separation region in part consists of polycrystalline silicon, and wherein the polycrystalline silicon is at a potential bias higher than the minimum potential of the element region.
 7. The device of claim 6, wherein the potential bias of the polycrystalline silicon is equal to a potential at which the second semiconductor substrate is held. 